This application claims the priority of Korean Patent Application Nos. 2003-72803, filed on Oct. 18, 2003 and 2004-72042, filed on Sep. 9, 2004 in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a wavelength division multiplexing passive optical network (WDM-PON) system, and more particularly, to a hybrid-integrated wavelength-tunable external cavity laser (ECL) light source technology required to form a PON based on a WDM method.
2. Description of the Related Art
It is predicted that it will be difficult for a conventional digital subscriber line (DSL) technology via an unshielded twisted pair (UTP) and a conventional cable modem termination system (CMTS) via a hybrid fiber coax (HFC) to provide a bandwidth and a quality guarantee enough to provide a service in which voice, data, and broadcasting are united to subscribers. To solve this problem, a fiber-to-the-home (FTTH) technology connecting to subscribers using optical fiber has been being developed all over the world.
According to expansion of digital home service, it is predicted that an average bandwidth of more than 100 Mbps per subscriber will be required. Therefore, it is predicted that a conventional DSL method and a conventional cable modem method in which it is difficult to provide a bandwidth of this level will be gradually substituted with an FTTH method.
Main problems that must be solved for the FTTH technology to be successfully applied include firstly that it can accommodate an increase of subscribers as much as possible without changing existing optical fiber installations and secondly that an expense required to connect to subscribers using the optical fiber must be low-priced in a level which can be compared with expensees by existing methods (DSL and CMTS for example).
The most important thing in a development of the FTTH technology is to develop a light source suitable for a subscribers network, and according to characteristics of the subscribers network, an economical efficiency, mass productivity, and easiness of installation and management must be most preferentially considered.
The optical subscribers network can be largely divided into a passive optical network (PON) method and an active optical network (AON) method.
The PON method has been being improved in types of asynchronous transfer mode PON (ATM-PON), broadband PON (B-PON), gigabit PON (G-PON), and Ethernet PON (E-PON). The AON method has been being improved in a type of connecting Ethernet switches to each other in a hierarchical structure using the optical fiber. In the PON method and the AON method, an optical transmission line is formed on a single wavelength per transmission direction. However, as for providing a high bandwidth of more than 100 Mbps, which guarantees quality to a subscriber, it is predicted that these methods will confront limitations in a use and effectiveness of an optical path between a central office (CO) and the subscriber. In order to solve this problem, an attempt of introducing a WDM technology to the subscribers network has been proceeding lately.
A WDM-PON method provides an effect of increasing physical links by multiplexing a plurality of optical wavelengths in a strip of optical fiber. Accordingly, since curtailment of expenditure due to reduced number of lines and a centralized operation management in a head-end is possible by accommodating a plurality of subscribers in a single optical transmission line, the first condition that FTTH technology must meet for a success can be satisfied. Also, since traffics of subscribers are separated by allocating an optical channel having a different wavelength per subscriber, advantages exist in security and protocol transparency.
For an FTTH network based on the WDM, various methods of transmitting an optical signal have been suggested till now. There are a method in which a light source is included in a subscriber and a loop-back method in which a modulator instead of the light source is included in the subscriber and light transmitted from a CO is only modulated and re-transmitted to the CO. When a light source is included in a subscriber optical network terminal (ONT), there are a method of using a light source whose wavelength is designated (for example, a distributed feed back laser diode (DFB-LD)) and an injection-locking method of generating an optical wavelength locked to a wavelength of an injected external beam by injecting the external beam from the outside into a broadband light source, such as a fabry perot laser diode (FP-LD).
When a light source whose optical wavelength is designated is installed to a subscriber, light sources emitting different optical wavelengths must be assigned to the other subscribers in a given network.
At this time, N light sources outputting different optical wavelengths λ1 through λn and one N×1 WDM multiplexer (WDM MUX) multiplexing the output optical wavelengths λ1 through λn are installed in a sending end, and one 1×N WDM demultiplexer (WDM DMX) dividing a received signal according to wavelengths and N photo diodes (PDs) reproducing an electric signal from an optical signal are installed in a receiving end. However, in this method, an inventory management problem that ONTs must be produced, managed, and installed to subscribers by different wavelengths is generated. That is, when the method of installing a light source whose optical wavelength is preassigned to a subscriber is massively applied to a market, the inventory management problem may be a big obstacle against market expansion.
In order to solve this inventory management problem, there are various methods of providing an optical beam having a wavelength designated by the CO to a subscriber. One of the various methods is a method where a incoherence broadband light source (e.g., erbium-doped fiber amplifier (EDFA)) is transmitted towards subscribers, divided into by a WDM DMX near the subscribers, injected into each of FP-LDs of the subscribers; one of the FP-LD modes is locked to the injected wavelength; and data are transmitted on the injection-locked mode of the FP-LD.
However, disadvantages of this method are firstly that an expensive EDFA as a broadband light source for an external injection beam must be used and secondly that it is difficult to transmit data larger than 1 Gbps because of temperature unstability and optical interference noise innate in this method.
In order to solve a physical volume problem of optical modules generated at the CO by accommodating a plurality of subscribers based on the WDM-PON, an array type multi-wavelengths light source module has been being briskly developed together with various light source methods. Conventional light source modules are mostly based on a DFB-LD type. However, many problems, such as complexity of a manufacturing process, yield degradation due to an increase of the number of wavelengths, and reliability for a stable operation, must be still improved.
As described above, important problems that the WDM-PON must solve are firstly to achieve lowering the price of optical module, secondly to achieve increasing integration density of optical modules at an optical line terminal (OLT) optical module, and thirdly to achieve an ONT independent on a specific optical wavelength.